Digestive system introduction

author/s: Dr Mª del Carmen Peña Cala

The function of the digestive system is to provide nutrients to all body cells.

To carry out its function, the digestive system first needs to perform a physical and chemical breakdown of the food into its essential components, to proceed to their absorption through the intestine. The last step is to eliminate the debris materials that have no use.

The process of digestion starts when the food enters the mouth and ends when the debris are eliminated in the form of faeces through the anus. During this process, food moves down and mixes with different secretions in the digestive tract thanks to muscular contractions called peristaltic movements that are produced along the digestive tract.

To function correctly, the digestive system needs a digestive tract and other digestive structures relating to it.

The digestive tract is formed by the mouth, pharynge, oesophagus, stomach, thin intestine, appendix, large intestine, rectum and anus.

Digestive structures related to the digetive tract are: salivary glands, páncreas, liver and gallbladder, each of these with a specific function.

Mouth

An opening in the anterior and inferior portion of the face through which food is taken to enter the digestive system.

The mouth houses the tongue and the teeth. Salivary glands pour their contents into the mouth. Mastication and salivation of food take place in the mouth, ending up in the formation of a food bolus that facilitates deglutition.

Pharynge

The pharynge (throat) is a canal through which air and degluted food pass as they move down to the oesophagus.

The pharynge connects through its anterior portion with the mouth and through the posterior portion with the larynge. The pharynge is separated from the larynge by the epiglottis and the oesophagus. Food is passed from the pharynge to the esophagus.

Oesophagus

A muscular tube formed by a thick wall of about 30 to 35 cm in length that receives the food bolus from the pharynge. The oesophagus pushes the bolus down towards the stomach by means of a series of peristaltic movements.

Stomach

Dilatation of the digestive tract that connects with the oesophagus through a sphincter called “cardias” and with the duodenum thorough the pylorus.

The stomach has different types of cells that play a role in the secretion of gastric juices.

"Trituration” or grinding of solid food and emptying towards the duodenum also takes place in this part of the digestive tract.

Small intestine

The small intestine starts at the pyloric sphincter at the end of the stomach and ends in the cecum, a bag-shaped region where the large intestine starts. It measures from 6 to 8 meters in length.

The small intestine consists of three sections: These three structures produce digestive enzymes that play a significant role in the breakdown and absorption of food.

Large intestine or colon

When digested food (chyme) reaches the large intestine, the majority of nutrients have already been absorbed. The most important function of the colon is to change chime into faeces to be excreted. During this process, the colon absorbs water from the chime, changing its liquid state to solid.

This duct measures about 1.50 cm in length and is formed by the following sections: CECUM, ASCENDING COLON, TRANSVERSE COLON, COLON, SIGMA, RECTUM and ANUS.

Cecum

Is like reservoir sac situated below the ileocecal valve that also houses the appendix.

Rectum

Is a 12 cm long structure forming the final section of the large intestine where the faeces accumulate before defecation.

Anum

The anal canal is a structure situated below the rectum that measures about 4 cm in length. The walls of the canal are formed by two concentric muscular layers called internal and external sphincters that act as valves, relaxing during defecation to expulse faecal matter outside.

From inside to outside, we find the mucosal layer in direct contact with the food bolus, the submucosal and serosa layers, excepting the oesophagus devoided of this last layer.

Salivary glands

Páncreas

The pancreas is an elongated gland situated behind the stomach and partially inside the curve of the duodenum. It is believed to be a mixed gland because it secretes hormones (endocrine component) and pancreatic juice (exocrine component).

The pancreatic juice empties into the duodenum.

Liver

The liver has a fundamentally metabolic function.

The liver is a large organ located in the right upper portion of the abdominal cavity. It is as a chemical processing plant that performs many different functions: it processes absorbed nutrients, stores glucogene, iron and some vitamins; it eliminates toxins and debris products of the blood, transforming them into less harmful substances; it produces bile and plays an important role in the digestion of fat.

Gallbladder

The gallbladder is a pear-shaped bag situated below the liver where the bile produced by the liver is stored.

Bile is a liquid, thick, greenish/yellowish substance with a bitter taste that empties into the intestine through the biliary ducts. It plays an essential role in the digestion of fat.